The present invention relates to methods for determining lightning stroke distances and, in particular, to the automation of such methods.
Because of the dangers and damaging effects of lightning, it obviously would be beneficial to have warning systems providing automatic alerts. For example, ammunition loading or assembly depots, as well as fuel-handling facilities clearly could benefit from such protection. Also, lightning can upset computer memories and disrupt the proper operation of telecommunictions and integrated circuit systems. However, as far as is known, simple and reliable automated systems presently are not available.
It would seem that the provision of such systems would be a relatively simple matter. For years human observers have figured the closeness of the flashes by counting the time interval between the flash and the sound. Since sound travels at approximately 340 meters/second, the distance value is readily available. Consequently, at first glance, automation of this `flash-bang` technique simply would involve only the use of readily-available acoustic and optical sensors coupled with a timer and a simple calculator. However, when the problems are analyzed, the solutions become somewhat complicated particularly if a relatively high degree of precision or reliability is desired. Thus, with regard to reliability, the human observer technique is somewhat uncertain in that there is no guarantee that the `bang` is the first sound produced by the lightning. Nor is there any certainty that the bang is even associated with one particular flash. Further, although the observer usually assumes that his calculation represents a horizontal or ground plane distance, the fact is that the sound is emanating from a point in the sky and that the observer actually is calculating a slant range.
More specifically, if the observer is basing the calculation upon a loud thunder clap, as contrasted with an initial rumble noise, it is well known that the origin of the clap is a shock wave propagated from a section of a lightning channel which is approximately perpendicularly oriented to the line of propagation of the sound to the observer (or the sensor). Consequently, since the origin may be at a considerable height above the ground, its slant range becomes at best a questionable estimate of the horizontal ground distance. Also, although the initial rumble rather than the load clap probably is the closest part of a lightning channel, often it is difficult to associate the rumble with a particular lightning event. The first load clap after sighting a cloud-to-ground lightning flash is more reliable. Again, however, studies have shown that such load claps may follow the initial sound by as much as 14 seconds. Thus, an early study by W. J. Remillard "Acoustics of Thunder" (Harvard University Tech, Memo #14 1960), plotted the numbers of thunder events against the time between the first thunder sound and the first loud clap after a cloud-to-ground flash was seen. The plot clearly indicates that the first loud clap may follow the initial thunder cloud, presumably the closest part of the channel, by rather long periods of time extending, as stated, up to 14 seconds. A flash-bang determination in such a case would locate the channel at a distance of approximately 4 to 5 miles further than it actually occurred relative to the observer.
Automatic lightning warning systems which resolve these and other problems in a simple and effective manner apparently have not been developed or, at least, as far as is known, are not available. Any such system at a minimum should provide a close estimate of the horizontal distance between points having a high probability of a lightning strike and it should assure to the maximum extent possible that the observed lightning event is the origin of the sound.